900MHz packet radio?

Paul Gardner-Stephen paul at servalproject.org
Thu Sep 1 15:03:36 UTC 2011

Hi Sylvain,

On Thu, Sep 1, 2011 at 10:51 PM, Sylvain Munaut <246tnt at gmail.com> wrote:
> You have to essentially stick with GSM mostly so:
>  - Same modulation GMSK
>  - Same frame structure of 4.615ms
>  - Same bursts : 156.25 bits
>  - Same channel coding for CCH and TCH
>  - Same channel spacing of 200kHz
> This way you can mostly reuse the GSM primitives offered by the baseband DSP.
> Then you have to create new primitives so that phones can do between
> them what they would do with a BTS
>  - Transmit a FCCH / SCH ( so that a phone can become 'local master'
> and provide sync to other phones )
>  - RX RACH requests ( so that a phone that's 'local master' can answer
> incoming requests for channels )
> Both of theses actually already exist (I coded them).

Most interesting!

> Then you have to come up with a protocol that with those primitives
> can build a stable mesh network.

Well, once there is a stable packet transport, then it is fairly easy
to build the mesh on top.

> And this latter part doesn't actually require any messing with
> osmocom-bb at first, you should design and simulate it fully _first_.
> And then once you have it you can start implementing it on real
> hardware.

Indeed. The Serval mesh software is being setup so that it can use any
link-layer, including such a GSMish one as you have described.  That
part is the easy part from my perspective.

> You could probably get some inspiration from the specs of TETRA DMO
> mode since what you want is essentially a DMO mode for GSM.

Yes, in some ways that is what we want. We also need to deal with
bridging to WiFi and some other odd bits and pieces.


> Cheers,
>    Sylvain

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